1. Field of the Invention
The present invention relates to improving the flow and pour point characteristics of middle distillate fuels. More particularly, the present invention relates to the preparation of hydrocarbon fuels, in particular heating oils and diesel fuels, with improved low temperature handling characteristics.
2. Description of the Prior Art
In storage and use of heavy oils, such as lubricating oils, problems associated with pour point have long been in existence and have been recognized in the art. The pour point of an oil is defined as the lowest temperature at which the oil will pour or flow when chilled without disturbance under specified conditions. Recently, it has been discovered that pour point problems also exist in the storage and use of petroleum fuel oils, particularly at low temperatures. Pour point problems arise through the formation of solid or semi-solid waxy particles within an oil composition. For example, in the storage of furnace oils or diesel oils during the winter months, temperatures may decrease to a point as low as -15.degree. to -40.degree. F. The decreased temperatures often cause crystallization and solidification of wax in the distillate fuel oil.
The low temperature properties of petroleum distillate fuels boiling in the range between about 250.degree. and about 800.degree. F. have become important in recent years because of the growth of markets of such fuels in subarctic areas. The increases in the use of hydrocarbon fuels of all kinds in areas frequently subjected to low temperatures have presented serious problems because of the poor cold characteristics of these fuels. Particularly serious problems have been encountered in the use of oils and diesel and jet fuels that have too high a pour point, resulting either in distributing or operating difficulties or both. For example, the distribution of heating oils by pumping or siphoning is rendered difficult or impossible at temperatures around or below the poor point of the oil. Further, the flow of the oil at such temperatures through the filters cannot be maintained, leading to the failure of the equipment to operate.
It is well known in the art to use various mixtures and additives in order to lower the pour point of oils. For example, a wide variety of compounds has been found to be effective as pour point depressants for lubricating oils. Among the best known are those prepared either by condensing aromatic compounds with long chain paraffins, or by condensing olefinic esters. It is generally considered that these pour depressants are effective because as an additive-containing oil is cooled, the hydrocarbon chain of the additive becomes incorporated into the crystal lattice of the separated wax, while the other part of the pour point depressant molecule prevents the crystals from adhering together to form a gel structure. It has, however, been found that such known pour point depressants generally have little or no effect in oils of the middle distillate boiling range. The failure of these additives to be effective in middle distillates may at least in part be due to the basic difference in the composition between the wax in lubricating oils and that in middle distillate fuels.
Middle distillate fuels have been found to be responsive to such materials as terpolymers derived from ethylene-propylene and a C.sub.1 -C.sub.12 alkyl ester of acrylic acid or methacrylic acid or mixtures thereof (see U.S. Pat. No. 3,447,915) and halogenated polymers of ethylene and C.sub.3 -C.sub.18 alpha-olefins.
U.S. Pat. No. 3,961,916 teaches that the low temperature flow characteristics of petroleum middle distillates can be very satisfactorily controlled by the proper choice of a combination of a nucleating agent or wax growth stimulator and a wax crystal growth arrester. This was based on discovery that depending on its composition and physicochemical characteristics such as molecular weight and branchiness, a polymeric wax crystal modifier can essentially operate as a nucleator or as a growth arrester for wax crystals. According to this patent, one convenient way of achieving this in a most effective way is to add a separate polymeric additive to effect each of the separate functionalities desired.